We have observed previously that several aspects of physiological and behavioral development are related to maturational changes in specific populations of brain and/or peripheral monoaminergic neurons. We have also learned that the neurochemical and behavioral potency of some psychoactive drugs change as a function of the age of the animal. Finally, we have been able to use a variety of surgical, pharmacological, and nutritional manipulations to alter brain and peripheral monoaminergic neurons in fetal and newborn animals. We now hope to continue studies that are designed to: 1) characterize more fully the age-related changes that occur in the maturation of brain and peripheral monoaminergic and cholinergic neurons in developing animals; 2) determine whether changes in these compounds are correlated temporally with maturational changes in physiological and behavioral development; 3) determine the extent to which various psychoactive drugs produce age-related changes in the physiology and behavior of animals; 4) examine some of the mechanisms that might underlie developmental differences in drug responses; and 5) determine whether various surgical or pharmacological manipulations that alter brain or peripheral neurotransmitters in young animals produce predictable changes in physiological and behavioral development.